Weld schedule for resistance spot welding of aluminum alloy workpieces

ABSTRACT

Aluminum-base alloy workpieces have surfaces with films of aluminum oxide which inhibit good contact with weld faces of resistance spot weld electrodes and the faying surfaces of, for example, sheet workpieces stacked for welding. Sometimes, the surfaces of the sheets also are coated with an adhesive or a sealer which further complicates welding. But in accordance with this invention, weld faces of opposing, round, copper welding electrodes are pressed against opposite outside surfaces of the sheets at a spot weld site and weld current is applied to the electrodes in accordance with a three-stage weld schedule to better form each weld. The weld schedule comprises a Conditioning stage (stage 1), a weld nugget Shaping stage (stage 2), and a weld nugget Sizing stage (stage 3).

This application is a continuation application which claims prioritybased on non-provisional application Ser. No. 13/309,825, titled WeldSchedule For Resistance Spot Welding Of Aluminum Alloy Workpieces filedDec. 2, 2011, which claims priority based on provisional application61/527,155, titled “Weld Schedule for Resistance Spot Welding ofAluminum alloy Workpieces,” filed Aug. 25, 2011, and which isincorporated herein by reference.

TECHNICAL FIELD

This invention pertains to improvement in the formation of resistancespot welds in a stack of aluminum-base alloy workpieces. Morespecifically, this invention provides a method utilizing the variedapplication of current flow values and times in such resistance spotwelding in three separate steps or stages to allow weld faces of theelectrodes to better engage the aluminum workpieces, to better initiateformation of the molten weld nugget in the center of the faying surfacecontact zones, and to quickly achieve the target weld nugget size.

BACKGROUND OF THE INVENTION

Resistance spot welding typically comprises pressing round weld facesurfaces of two opposing, high conductivity, copper electrodes againstopposite sides of two or three overlapping metal sheets (sometimescalled a “stackup”), and passing an electric current for a period ofseveral milliseconds to a few hundreds of milliseconds between theelectrodes through the sheets to form a weld nugget at thesheet-to-sheet interface, called the faying interface.

Resistance spot welding of aluminum workpieces (typically aluminum-basedalloys containing 85% or more by weight aluminum) at high volume isconsidered to be very difficult within the automotive industry becauseof several issues. The workpieces are often rolled aluminum alloy sheetmaterials, but may also be extrusions or castings of suitablecomplementary shape for spot welding. While complementary-shapedaluminum-based alloy body panels may be placed together and joined by aseries of suitably located spot welds, the aluminum workpieces may be ofequal or different thicknesses, of the same or different aluminumalloys, coated on the surface, and may have adhesives or sealantsapplied along weld flanges. There may be small gaps between theassembled panels and one or both of the opposing welding electrodes maybe positioned at an angle slightly different from its intended weldingposition.

One of the major issues is the presence of a tough, adherent,non-conducting oxide film on the aluminum substrate surface. This oxidefilm can cause excessive overheating at both electrode/sheet interfacesas well as the sheet-to-sheet faying interface. Typical solutions to theproblem of electrode/sheet interface overheating include the use ofelectrodes designed with large, flat weld faces that reduce the currentdensity and, thus, heating at these locations. The use of large, flatelectrodes produces undesired consequences for manufacturing. Thesetypes of electrodes are 1) sensitive to gaps between workpieces, 2)sensitive to electrode orientation, i.e., being off-angle or off-normalwith respect to the workpiece surface, and 3) require large flanges onthe workpieces to accommodate the large electrode body diameter and weldface on the electrode.

Several electrode designs and dressing processes that address theseissues may be found in patents and patent applications, including one orboth of the inventors herein and owned by the assignee of thisinvention: U.S. Pat. No. 6,861,609 (Mar. 1, 2005) and U.S. patentapplications published as 20100258536, 20090302009, 20090255908,20090127232, 20080078749. The problem of the oxide film and resultantelectrode/sheet overheating has been addressed by placing geometricfeatures, such as a micro texture or a series of ridges and grooves onthe weld face that, under weld load, penetrate the oxide layer onaluminum to lower contact resistance and heat generation at thatinterface. The reduction in electrode/sheet heating has two directbenefits. First, it allows a smaller electrode with less thermal mass tobe used, which decreases flange requirements. Second, it allows asharper electrode weld face curvature to be used that betterconcentrates welding current. This makes the welding process much lesssensitive to both the electrode orientation on the workpiece, i.e., theelectrode being off-angle with respect to the workpiece, and thepresence of gaps between workpiece surfaces.

Despite these very significant improvements in process performanceattained by solving the issue of high contact resistance at theelectrode/sheet interfaces, issues remain to be solved before thealuminum spot welding process is considered sufficiently robust for highvolume manufacturing. Many of these issues are related to the presenceof surface oxide films at the sheet-to-sheet or faying interface, whichis unaffected by modifications to the electrode weld face. These issuesare related, in part, to the nature of the direct current weldingprocess typically used in automotive aluminum welding which is termedMedium Frequency Direct Current or MFDC. This process uses an invertertype weld control that receives a three phase, 60 Hz alternating currentpotential at 480 volts rms (in the United States) and provides a singlephase square wave of higher voltage, about 650 volts, to the MFDCtransformer at a frequency of approximately 1000 Hz. The transformerreduces the high voltage waveform supplied by the weld control to a muchlower welding voltage (for example 13 volts at a 50:1 transformer turnratio) at much higher current. The low voltage, square wave output atthe transformer is then rectified with high current capacity diodes toprovide DC current for delivery to the welding electrodes and stackup ofworkpieces. During setup for producing many like welds on a series ofworkpieces, a suitable weld current and weld time are predetermined. TheMFDC weld controller is then programmed to deliver a nearly constantcurrent (e.g., twenty-five to thirty kilo amperes) to the weldingelectrodes pressed against a workpiece over a weld cycle of about 250 to300 milliseconds. In any DC process, and particularly one where currentflow is programmed to be nearly constant, as with MFDC, one electrode(positive) runs considerably hotter than the other electrode (negative)when in contact with aluminum substrates. This temperature bias of theelectrodes can affect weld nugget formation and growth, especially forstickups of sheet workpieces that are asymmetrical with respect to boththickness, i.e., high thick/thin ratios, and material, e.g., weldingAluminum Alloy 5754-O to Aluminum Alloy 6111-T4 sheet. AA5754-Ocomposition limits are 2.6-3.6% Mg, <0.4% Si, <0.5% Mn, <0.4% Fe, and<0.1% Cu (balance substantially all aluminum), while age hardenableAA-6111-T4 composition limits are 0.5-1.0 Mg, 0.6-1.1% Si, 0.1-0.45% Mn,<0.4% Fe, and 0.5-0.9% Cu. This can result in stackups welding better inone orientation relative to electrode polarity than the other includingproducing larger welds or greater weld penetration in one orientationthan the other, which in production operations would not be ideal. Inaddition, the hotter running positive electrode is more prone to wearand, thus, can shorten electrode life by requiring more frequentdressing.

In addition to the polarity effects, the standard constant current weldschedules that have been used in production applications of aluminumspot welding can produce other undesired issues. These schedules arebased on the application of a constant current, e.g., 27 kA, over a settime, e.g., 200 milli-seconds (ms), at a constant force of theelectrodes against the workpiece surfaces. The issues that have beendiscovered include excessive electrode wear, sensitivity to weld spacingfor heavy gauge stackups, inconsistent size and quality of the firstweld, weld microstructures that lead to undesirable weld fracture modes,and weld shapes that lead to premature expulsion and poorer weldquality. The undesirable fracture modes, which occur in peel or tensileloading, include weld fracturing along the faying interface orfracturing around the weld nugget perimeter and, thus, not forming abutton that pulls completely through the sheet thickness. Finally, thestandard constant current weld schedules are less robust in the presenceof sealers or adhesives. With adhesives or sealers present, theseschedules tend to result in nuggets with more defects that tend tofracture in undesirable modes especially when subject to peel loading.

There remains a need for improved practices for resistance spot weldingof aluminum alloy sheet metal workpieces and other workpiece shapes.

SUMMARY OF THE INVENTION

Practices of the invention will be described using a stackup (anassembly) of two or three aluminum-based alloy sheet workpieces. Sheetworkpieces often have thicknesses in the range of about one-half toabout four millimeters and a stackup for welding may be formed of thesame or different aluminum alloys and of different thicknesses. In manywelding operations, more than one weld may be formed simultaneously on astackup of sheets, and hundreds of welds may be formed on many stackupswith a welding apparatus setup during a working shift. It is necessaryto manage the welding equipment and process to reliably and repeatedlyproduce uniformly good welds on aluminum alloy workpieces.

Embodiments of the invention are based on the discovery that good weldsmay be formed more reliably on aluminum workpieces by varying thewelding current in distinct welding stages or steps during the formationof each resistance spot weld. In accordance with embodiments of theinvention, welding current is applied at different levels during threespecific steps of each spot weld cycle with cooling times or “off” timesbetween each step. The equipment for delivery of the direct currentwaveform to the welding electrodes usually comprises a weld gun witheither high-conductivity arms or high-conductivity cables for deliveringthe current to the electrodes, a MFDC transformer, and an inverter typeprogrammable electronic control. To provide the proper waveform forwelding automotive gauge aluminum workpieces, the welding system, i.e.,programmable inverter weld control, MFDC transformer, and weld gun, mustbe able to provide high current output and fast rise times delivered tothe welding electrodes at a programmed schedule determined during set upof the weld job. Such members of this welding system are commerciallyavailable. Current rise times measured at the electrode shouldpreferably be on the order of ˜10 ms to achieve 40 kA or ˜4 kA/ms.Significantly slower rise times will not be able to produce the desiredwaveforms satisfactorily and welding performance will be compromised.Each part of the weld gun system, i.e., control, transformer, and gun,affects both the maximum current output and the current rise time.

The first stage of our spot welding process is considered a Conditioningstage or step. It begins as two opposing welding electrodes have beenpositioned to engage opposite sides of a stackup of, for example, twosheet workpieces with a predetermined stable clamping force. Theelectronic control is operated to limit the voltage delivered by thewelding transformer. This can be accomplished by putting theprogrammable weld control into a suitable automatic voltage compensationmode (AVC) or similar mode. As an alternative to the voltagecompensation mode, the Conditioning current can be programmed inconstant current mode with a gentle slope over 10 ms or more from a lowcurrent value (˜3 kA) up to the final desired Conditioning current.Again, the objective is to obtain a consistent, stable resistance at theweld site, while not allowing the sheets to melt at the fayinginterface. The current is brought up gradually over a few millisecondsto a level for decreasing the electrical resistance to a consistent lowvalue at both the electrode/sheet interfaces and the underlying fayingsurfaces. In many weld setups the current increases from about 3 kA toabout 10 kA. In general, it is preferred to adjust the current to alevel just below that at which substrate melting occurs. Current flow inthis Conditioning stage is maintained for about 20 ms to about 60 ms.The current flow is suitably maintained for a time to obtain a stableand consistent current level.

During this Conditioning step, the electrode/workpiece interface isheated; permitting shaped features of the electrode weld faces to makebetter contact through the oxide coatings and with the metal workpiecesurfaces. The resistance as measured between the electrodes decreasesand the setup is now better prepared for molten weld nugget initiationand growth. Current flow is turned off by the weld control for about 10ms to cool the electrodes preparatory to the next stage in this spotweld process. This length of off time allows the current level to decayto zero. Longer cool times could be used, but would only decrease welderthroughput.

The second stage of our spot welding process is a weld nugget Shapingstage. This Shaping stage is designed to initiate formation of a good,round, centered (on the axis of the opposing electrodes), molten weldnugget in spite of the polarity of the weld electrodes, or slightly offangle alignment of the electrodes to the workpiece, or non-ideal fitupof the workpieces, or the presence or absence of non-metallic adhesivesor sealers at the faying surfaces. This is accomplished by causing thecurrent to flow at a relatively high value (e.g., between 20 kA and 50kA) over a relatively short period of time, for example, a minimum timeof about six to ten milliseconds and suitably about six to fiftymilliseconds. The high target current and short times require use ofsuitable welding system components. As discussed previously, to achievethe target shaping current over these short times, the inverter weldcontrol, MFDC transformer, and weld gun must be appropriately designedto be able to achieve the target current and current rise time. Slowrise times prevent the attainment of the target current within the smalltime window of the Shaping step, which is necessary for the function ofthis step. Slow rise times can be due to slow weld control hardware orsoftware, insufficient MFDC transformer voltage, or high weld guninductance. The purpose of this high current step is to initiateformation as rapidly as possible of a liquid (molten) weld nugget in thecenter of the faying interface contact zone. This stage is kept shortwhile seeking to commence formation of a weld nugget, suitably about atleast three millimeters in diameter. The current level is kept lowenough to avoid sticking between the electrodes and workpieces and toavoid expulsion of metal at the electrode/sheet interface. Thedetermination of the current and time for a set up of aluminumworkpieces for welding may be determined by experience and or trials.Expulsion at the faying interface can occur, but since the molten nuggethas not yet fully formed, it causes no damage to the final nuggetstructure or properties. If it is found necessary to use a shaping timegreater than about thirty milliseconds, it is preferred to use twoshorter current pulses with an intermediate cool or off time to reducecurrent.

Once nugget shaping has been accomplished, the current is turned off bythe controller for about five milliseconds to allow it to decay. Thepurpose of such current off time is two-fold. First, it allows for somecooling at the electrode-sheet interface. Second, it prevents theinitiated molten weld nugget from overheating. Removing this coolingstep typically leads to overheating during the sizing step and severeinterfacial expulsion. Allowing the initiated nugget to cool helps bringthe process under control. However, this cooling step cannot be so longas nugget solidification occurs. If the nugget were to solidify it wouldact as a short between the sheets and further current flow during thesizing step would not achieve the desired weld size.

The third stage of the welding process provides for growth and sizing ofthe now-initiated weld nugget. This Sizing stage is initiated while thenugget is still in a molten state. A lower weld current is used in thisSizing stage than in the nugget Shaping stage. For example, a weldcurrent of about 15 to about 40 kA may be suitable in this third stage.The sizing time is determined based on the thickness of the thinnestsheet in a two sheet stackup and on the thickness of the second thinnestsheet in a three sheet stackup. A weld nugget of suitable size, e.g.,about six millimeters in diameter for a two millimeter thick sheet, isproduced. The current is adjusted to achieve a desired weld nugget size.And the duration of this stage is the longest of the overall weldschedule, often requiring from about twenty milliseconds to abouttwo-hundred milliseconds. If welding electrode wear is excessive, it maybe preferred to break current flow into pulses of about ten to aboutthirty millisecond durations of heating (on time) with short cool times(off time) of one millisecond to ten milliseconds.

Apart from the brief cool times between the spot weld stages, the weldprocess is continual and typically requires less total weld time(combined times of step 2 and step 3) than conventional MFDC or AC spotwelding of aluminum. It is found that the three-stage weld schedule ofthis invention more reliably produces good welds, and this result isattained over the formation of many welds. Further, less power isconsumed.

Other objects and advantages of this invention will be apparent from adetailed description of preferred embodiments which follows in thisspecification. Reference will be made to drawing figures which aredescribed in the following section of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view illustration of a welding systemincluding weld electrodes and two aluminum alloy sheet workpieces suchas may be used in practicing welding methods of this invention.

FIG. 2 is a graph of weld current (dash-dot line), voltage (short dashline), power (dash-dot-dot line) and resistance (solid line) as afunction of time for a stackup of a 2.0-mm thick AA 5754 sheet to a2.0-mm thick AA5754 sheet, welded with a conventional, constantcontinuous current resistance spot weld schedule.

FIG. 3 is a graph showing weld sizes for a 2.0-mm 5754 stack-up weldedunder three different conditions using a constant continuous currentschedule. Weld size is shown as a function of weld time for aconventional constant continuous current schedule. Weld sizes are shownfor coupons in three conditions, i.e., flat coupons or coupons orientednormal to the electrode (called “flange” in FIG. 2 with diamond datapoints), coupons 6° off-angle to the electrode (triangle data points),and coupons with a 0.8-mm gap (square data points). Open symbolsrepresent samples with undesired fracture modes, i.e., face tears orhalf buttons.

FIG. 4 is a graph of weld current (dash-dot line), voltage short dashline), power (dash-dot-dot line), and resistance (solid line) as afunction of time for a 2.0-mm AA5754 to 2.0-mm AA5754 stackup weldedwith the Conditioning, Shaping, and Sizing (CSS) schedule of thisinvention.

FIG. 5 is a graph of weld size as a function of weld time for aConditioning, Shaping, Sizing weld schedule. Weld sizes are shown forcoupons in three conditions, i.e., flat coupons (flange, diamond datapoints), coupons 6° off-angle to the electrode (triangle data points),and coupons with a 0.8-mm gap (square data points). Again, open symbolsrepresent samples with undesired fracture modes, i.e., face tears orhalf button.

DESCRIPTION OF PREFERRED EMBODIMENTS

The weld schedule of this invention may be practiced using weldingequipment such as that illustrated schematically in FIG. 1. FIG. 1 inthis specification is also presented as FIG. 1 and describedsubstantially as follows in our co-pending U.S. patent application,published as U.S. 2011/0266260 and also as FIG. 1 as described in U.S.Pat. No. 6,861,609 of one of us.

In FIG. 1, a representative spot welding gun apparatus 10 withassociated equipment (e.g., items 30, 32, 34, 36, 38, 40, and 42described below) utilized in spot welding apparatus for aluminum-basedalloy(s) workpieces is illustrated. In such welding operations, anassembly of two or more aluminum alloy sheet panels 12 and 14 to bewelded is prepared and delivered by a conveyor or other device to thewelding gun apparatus 10. The welding gun apparatus is typically mountedon a robot which moves the welding gun apparatus along the edges (e.g.,flanges for welding) of aluminum alloy sheet panels 12, 14 to rapidlyaccomplish a succession of individual electrical resistance spot welds.

In FIG. 1, the metal panels 12 and 14 are shown poised between a pair ofaxially aligned and opposing electrodes 16 and 18 of a welding gun arm20. The gun arm 20 is in the configuration of a C so that the weld faces(52 for electrode 16) of opposing electrodes 16 and 18 can be brought tobear and press upon opposite sides of the aluminum alloy panels 12 and14. In the arrangement shown, electrode 16 is mounted on a shank 17which is inserted in a holder 22 attached to a fixed arm 24 of thewelding gun arm 20. The opposing electrode 18 is mounted on a shank 19and inserted in another holder 26 carried on an air cylinder or servomotor 28. Air cylinder or servo motor 28 is adapted to axially move theelectrode 18 into clamping engagement with the outer surface of thepanel 14. A source of high pressure air from a remote source, not shown,delivers air through a programmable air regulator 30 through air line 32to the cylinder 28 to provide clamping force. Alternatively, aservo-motor control provides current and voltage to the servo motor toprovide clamping. During a spot welding sequence, the timely applicationof air pressure to the air cylinder 28 or movement of the servo motoradvances holder 26 so that electrode 18 presses the sheets 12 and 14against stationary electrode 16 with a force in the order of 500 to1,500 pounds.

Weld gun 20, typically mounted on the end of a robot arm, is connectedto a robotic controller 34. Robot controller 34 manages and actuates theprogrammable air regulator 30 and also actuates a programmed weldcontroller 36. Weld controller 36 regulates the passage of primarywelding current to the welding transformer, which supplies current tothe electrodes. On command of the welding controller 36, primary currentis delivered through primary current line 38 to weld transformer 40.Weld transformer 40 converts the primary AC current to a lower voltage,higher AC current secondary welding current which is then rectified bysuitable diodes into a DC current and provided through a secondarycurrent line 42 and electrode holder 26 as well as conductive gun arm 20and electrode holder 22.

When a welding operation is being considered for a new assembly orstackup of aluminum alloy workpieces, such as aluminum sheets 12, 14 inFIG. 1, actual testing or previous welding experience is usuallyconsidered in order to prepare a welding gun assembly and to program therelated robot weld gun controller (e.g., controller 34 in FIG. 1) forclosing weld guns at a predetermined clamping pressure at a weld site,or series of weld sites, on a stackup of workpieces, and for determiningthe programming of the weld controller 36 for operation of weldtransformer 40 and for delivery of a suitable sequence of DC weldcurrents in accordance with the Conditioning, Shaping, and Sizing weldsteps of each weld formed in accordance with practices of thisinvention.

The method for determining the welding currents for Conditioning,Shaping, and Sizing steps are given below. This is done typically withcoupons that represent the metal stackups to be welded. Current,voltage, power, and resistance at the electrodes are monitored usinginstrumentation separate from the weld control. For the Conditioningstep, current is adjusted current to a level just below that at whichsubstrate melting begins to occur. Once this current level isdetermined, time is adjusted to insure that a stable, consistentresistance has been achieved between the two electrodes. A short cool oroff time is typically inserted after Conditioning to allow the weldcontrol software to reset. For the Shaping step, current is adjusted atminimum time (˜6-10 ms) by raising the current level until a round,centered nugget forms without substrate/electrode reaction, i.e.,sticking electrode to substrate or melting of the substrate surface. Ifsubstrate/electrode reaction occurs during Shaping, the Shaping time isincreased in small increments and concurrent with current adjustments toinitiate a satisfactory nugget without substrate/electrode reaction.Nugget initiation should include the formation of a fused area greaterthan 3 mm in diameter near the center of the contact patch formed at thefaying interface of the sheets. For Shaping times beyond ˜30 ms theShaping current pulse should be broken into two shorter pulses with ashort cool between them. Because of the high current levels achieved inthe Shaping step, once the nugget is shaped, a short cool or off time of˜5 ms is inserted to allow the electrode/sheet and faying interfaces tocool. The Sizing time is set as a function of the Determining Thicknessof the stackup. For a two sheet stackup the thinnest sheet is theDetermining Thickness. For a three sheet stackup the second thinnestsheet is the Determining Thickness. Sizing times vary between about 30ms for a 1.0-mm Determining Thickness to about 100 ms for a 3.0-mmDetermining Thickness. Once the Sizing time is determined, current isadjusted to achieve the desired weld size. If electrode wear isexcessive, the Sizing continuous current pulse is modified to includeheat and cool pulses, typically 10 ms to 30 ms of heat with 1 ms to 10ms of cool.

Commercial robot controllers are available and may be selected formanaging the opening and closing of welding electrodes with respect tothe workpieces. Also there are commercial programmable weld controllersavailable for the practice of the subject three-step weld process ofthis invention. Following are required characteristics of theprogrammable weld controller, associated transformer, electricalconnections, and weld guns with their weld electrodes.

The inverter weld control is programmable so as to be loaded withpredetermined instructions for the conduct of the Conditioning, Shaping,and Sizing steps of this weld schedule. Further, the programmableinverter weld control needs to have sufficient primary currentcapability to achieve the desired secondary current based on thetransformer turns ratio. For a typical transformer turns ratio of 50:1,the inverter weld control requires a primary current capacity of atleast 1000 amp and preferably 1200 amp to achieve the target 50 kAsecondary current (turns ratio times max primary current) desired toreproduce these weld schedules for the thickest aluminum sheets. Weldcontrols are readily available with these primary current capacities.The weld control can also affect the secondary current rise times eitherthrough the hardware or software used by the control. Some controlslimit the rate of current rise either through design or simply the lackof speed of the internal processors or software. Assuming a typicalturns ratio of 50:1 for the transformer (other turns ratios of 40:1 to60:1 are available) experience has shown that the weld control primarycurrent rise time should be at least 80 A/ms or 10 ms to achieve 800 ampoutput, which would translate to a 4 kA/ms rise time for a transformerwith a turns ratio of 50:1.

As described above, the Conditioning, Shaping, and Sizing weld scheduleis developed by external monitoring of the current, voltage, power, andresistance at the electrodes, The weld control typically monitors onlycurrent, usually primary current and occasionally secondary current.Voltage, resistance, and power are not monitored by the weld control.The purpose of the weld control is to reproduce the desiredConditioning, Shaping, and Sizing (CSS) wave forms once they have beenestablished by the above set-up procedure.

The MFDC transformer should also meet minimum requirements. Thisincludes achieving the target maximum secondary current for the desiredwaveforms. For welding automotive structures the target maximum currentis typically about 50 kA while for welding lighter gauge closure panelsthis target is typically 35 kA. Output from both the transformer coreand diodes need to be capable of achieving these current levels. Forexample, a transformer with a 40:1 turns ratio in its core would notachieve the desired 50 kA output for structural welding using either a1000 amp or 1200 amp inverter control. Also, some transformer diodes mayhave limits on current levels that can make the transformer unsuitablefor this CSS three step weld process.

Assuming that the weld control produces both sufficient primary currentand primary current rise times to supply the transformer and thetransformer core and that the diodes have sufficient current capability,then the transformer output is controlled by two final factors:resistance and inductance of the weld gun arms and/or cables. Whileevery aspect of the welding system may be adequate for producing thedesired waveforms, if gun resistance and inductance are not satisfactorythen both maximum current and current rise time are compromised. Gunresistance is suitably measured along the gun arm between the electrodesand transformer. For example, typical resistance measurements may give50μ-ohm per arm with a total of 100μ-ohm for both arms. This level ofresistance will allow a 1200 amp inverter weld control with a 50:1 turnsratio MFDC transformer to achieve 60 kA. Higher resistances will beginreducing the maximum current that can be achieved. Secondary currentrise times are affected by gun inductance, which is directly related tothe loop area formed by the gun arms and/or cables. Large loop areas actto slow the current rise. Preferably, loop area should be less thanabout 300 square inches, e.g., 10 inches by 30 inches, for a 50:1 turnsratio transformer. This transformer has a maximum voltage of 13 volts.Larger loop areas would require transformers with higher voltages orlower turns ratios, e.g., a 40:1 turns ratio with 16.25 volts. Tosummarize, the entire welding system must be designed to achieve therequired weld currents and rise times, which requires the correct weldcontrol, MFDC transformer, and weld gun capabilities.

To achieve the desired waveform at the weld site, a relatively highcurrent (in kilo-amperes, kA) is delivered to the electrodes at arelatively low voltage over a period of milliseconds, typically lessthan 200 ms. The welding system is capable of delivering the requiredvoltage and current to the electrodes. The current, voltage, electricalresistance and power consumed at the electrodes are typically measuredindependently at the weld site. The weld controls can be used to measureprimary or secondary current. Current measurements, either measuredindependently or by the weld control, are made using a single turn coilaround wither the primary or secondary current lines. Voltage ismeasured independently by claiming voltage directly to the electrodes.From the current and voltage measurements, resistance (R=V/I) and power(P=VI) are calculated. Measurements of current, voltage, power, andresistance are made during welding of coupons designed to represent theactual metal stackup. In accordance with preferred embodiments of theinvention, the weld faces (for example 52 in FIG. 1) of the weldingelectrodes each present a spherical surface with concentric protrudingrings for engagement with the aluminum oxide films on surfaces of thesheet workpieces, although electrodes with micro-textured weld facesshould also benefit from this invention.

Before further illustration of the three step weld process of thisinvention it may be useful to describe the constant DC current practicenow used in electrical resistance welding of aluminum-base alloy sheetworkpieces. FIG. 2 shows a typical constant continuous current (CCC)weld schedule used to spot weld 2.0-mm Aluminum alloy 5754 sheet to2.0-mm 5754 sheet at an electrode force of 1200 lb. This amounted to afirst weld between the sheet materials without a previous shunt path.This type of schedule was used to weld components for GM's aluminum EV1vehicle produced some years ago. A similar, schedule of 167 ms iscurrently recommended by the Aluminum Association in their referenceWelding Aluminum Theory and Practice. The current profile (dash-dotline) features a rise from 0 ms to welding current (27 kA) over severalms (milliseconds). The slope of the current rise is typicallyunimportant for these schedules since once the target current isattained it is held for a relatively long weld cycle time. Current riseis followed by holding the current for about 200 ms. Current is thenshut off and falls to zero over several ms. The corresponding resistance(solid line) and voltage (short dash line) profiles both feature highinitial peaks. The high voltage peak at the start of current flow, about1.0 volt, results in arcing at the electrode/sheet interface and damageto the electrode weld face.

At the start of current flow for the CCC weld schedule, the voltageinput is fairly high, but the current level is low. This combinationdoes not provide sufficient power (dash-dot-dot line) early on toreliably initiate a weld nugget. The weld nugget may be initiated laterin time or may initiate in random areas throughout the faying interfacecontact zone established by the closed welding electrodes.

Delayed weld nugget initiation may impact weld size and result in thewide variation of weld size that is observed for conditions that includehigh thickness ratios, dissimilar alloys, and variable weld spacing. Italso drives the need for longer weld times to insure that the weld willbe appropriately sized under a variety of conditions, i.e., with gapspresent or welded off-angle.

FIG. 3 shows weld sizes for a 2.0-mm 5754 stack-up welded under threedifferent conditions using a constant continuous current schedule. Notethat weld initiation is delayed for samples that are oriented 6°off-angle with respect to the electrode. This necessitates longer weldtimes to achieve satisfactory performance, i.e., a weld size of ˜6 mm.Undesired fracture modes (open symbols) occurred out to weld times of 65ms and weld sizes above 5 mm further demonstrating the lack ofrobustness of this schedule.

Nugget initiation that is not focused and located in the center of thefaying interface contact zone, but is distributed randomly throughoutthe faying interface contact zone may cause poor weld penetration andsubsequent undesired weld factures and poor weld shape which can lead toexpulsion and poor weld quality. Random nugget initiation would alsoresult in poor weld quality for locations with adhesives or sealerspresent since initiation in several small locations spread throughoutthe adhesive or sealer would most likely entrain a greater volume of thesealer or adhesive that produce undesired porosity in the weldmicrostructure.

This invention is a weld schedule that utilizes three separate steps forproducing high quality welds with consistent size and shape over a widerange of conditions. The three distinct steps include Conditioning,Shaping, and Sizing (CSS). FIG. 4 is a graph of weld current (dash-dotline), voltage (short dash line), power (dash-dot-dot line), andresistance (solid line) between the electrodes as this three-stage weldschedule is applied to a stackup of a 2.0-mm thick AA 5754 sheetworkpiece to a 2.0-mm thick AA5754 sheet.

Step 1—Conditioning: This step begins once the electrodes have achieveda stable weld force clamping the stackup at a weld site. The weldcontrol is placed into a mode that regulates or limits the voltageapplied by the welding transformer and is referred to as AutomaticVoltage Compensation (AVC mode). FIG. 4 illustrates that during theConditioning step the voltage at the electrodes starts at about 0.5 voltand drops to 0.4 volt. Weld current is brought up gradually to a levelthat decreases resistance at both the electrode/sheet and fayinginterfaces to a consistent, low value. The current typically increasesfrom a low value of ˜3,000 to between 5,000 and 10,000 amps. In general,the exact current level is chosen to be just below that at which meltingoccurs at the faying interface. Also, current flow is maintained between20 ms and 60 ms. A minimum of 20 ms is required to insure that theresistance has broken down and stabilized sufficiently. Current flowbeyond ˜60 ms will typically not reduce resistance further, but onlyslow the welding process. For the example in FIG. 4, conditioning isdone for 50 ms and reaches a current of ˜10 kA. As an alternative to thevoltage compensation mode used by the weld control, the Conditioningcurrent can be programmed in constant current mode with a gentle slopefrom a low current value (˜3 kA) up to the final desired Conditioningstep current. Again, the objective is to obtain a consistent, stableresistance at the weld site while not allowing the sheets to melt at thefaying interface.

During the Conditioning step heat is generated at the weld site, whichallows the electrode's rings to establish more intimate contact at theelectrode/sheet interface. Improved contact prevents excessive electrodedamage from occurring during the shaping step where a high current levelis applied. In addition, the resistance as measured between theelectrodes decreases to a consistent value. In this example, FIG. 4, itis approximately 0.04 milli-ohms. For a given stackup, such as the onecited above, this consistent resistance value should be the sameregardless of weld gun polarity or the presence or absence of anyadhesives or sealers. This step insures that the current flow thatoccurs in the shaping step produces consistent weld size results. At theend of the Conditioning step, current flow is shut off for ˜10 ms toallow the electrodes to cool prior to the Shaping step. This is shown asa drop in current and voltage at 50 ms in FIG. 4. Longer cool (or off)times could be used, but would only slow the welding process. Dependingupon the weld control operation, the cooling step also allows the weldcontrol to reset to provide maximum power at the start of the shapingstep.

Step 2 Shaping: This step is designed to produce consistent molten weldnugget initiation, preferably centered between the opposing weld facesof the electrodes, regardless of the welding conditions, i.e., gunpolarity, presence or absence of adhesive/sealer, poor face-to-faceengagement of the sheet metal workpieces at their faying surfaces,off-angle electrodes, etc. Typically, current is forced to flow at avery high value to initiate a molten weld nugget as rapidly as possiblein the center of the faying interface contact zone. Obtaining very highcurrents in short times places high demands on the welding system. Asdiscussed previously, to achieve the target shaping current over theseshort times, the inverter weld control, MFDC transformer, and weld gunmust be appropriately designed to be able to achieve the target currentand current rise time. Slow rise times prevent the attainment of thetarget current within the small time window of the shaping step, whichis critical for the function of this step. Slow rise times can be causedby slow weld control hardware or software, insufficient MFDC transformervoltage, or high weld gun inductance. FIG. 4 shows that the rise time to35 kA is only 6 ms or 5.8 kA/ms, which is very fast. The Shaping pulseprovides a very high power impulse to the stackup. For the example inFIG. 4, power is seen to peak at 35 kW at roughly 70 ms. Compare this tothe peak achieved in FIG. 2 of only 22 kW.

Depending upon the sheet stackup, Shaping step current flow times of 6ms to 50 ms are used, with peak current values typically between 20 kAand 50 kA. Assuming that the welding system, i.e., inverter control,MFDC transformer, and weld gun, can achieve the desired current risetimes of >4 kA/ms, the shaping peak current and shaping time aredetermined within a couple of constraints. Shaping time is kept as shortas possible to obtain the desired initial molten weld nugget, which istypically at least ˜3 mm in diameter. Since, inverter design,transformer voltage, and welder inductance affects current rise time, itusually takes ˜5 to 12.5 ms to reach the target peak current level of 20kA to 50 kA. Shaping times are at least 6 ms, but typically greater than10 ms. Shaping should be no more than 50 ms. For thin sheets such as1.0-mm AA5754-0, low shaping currents are used such as 24 kA that can beachieved in a short time such as 6 ms, see Table below. Medium gaugesheet, such as 2.0-mm AA5754-0 may require 20 ms of shaping time. Forheavier sheets such as 3.0-mm AA5754-0, longer Shaping times are needed.In this case 50 ms of Shaping time is used. To prevent excessiveelectrode/sheet reaction the shaping time is broken into a 20 ms pulsefollowed by a short cool time and then a 30 ms pulse. In general, whenShaping times are required above 30 ms to produce a satisfactory weldinitiation site, then the Shaping pulse is broken in two with a short ˜3ms inserted between the two pulses. Peak current levels and weld timesare kept below those that either create excessive reaction between thesheet and electrode weld face in the form of sticking or result inactual melting of the sheet exterior surface.

The following Table 1 presents illustrative times for Conditioning,Shaping, and Sizing for the specified Sheet 1 and Sheet 2 Aluminum Alloycompositions and thicknesses. Column 4 illustrates Shaping peak RMScurrent values for respective sheet combinations. The last columnpresents the recommended weld times in milliseconds of the AluminumAssociation.

TABLE I Conditioning, Shaping, and Sizing weld parameters Shaping peakAluminum current minus Association* Conditioning Shaping Sizing RMSSizing recommended Sheet 1 Sheet 2 time (ms) time (ms) current (ka) time(ms) weld time (ms) 1.0-mm 1.0-mm 40 6 3 30 133 5754 5754 1.0-mm 2.5-mm40 20 3 60 133 5754 5754 1.5-mm 1.5-mm 40 16 4 80 166 5754 5754 2.0-mm2.0-mm 40 20 10 90 166 5754 5754 2.5-mm 2.5-mm 40 15 + 20 10 100 2505754 5754 3.0-mm 3.0-mm 60 30 + 20 9 100 250 5754 5754 *Welding AluminumTheory and Practice, Aluminum Association Inc. , June 1991, pg. 13.3

Once sufficient shaping has been accomplished, the weld control stopsdriving the current for ˜5 ms. During this time period, current fallsrapidly until it is beneath the level used for sizing, but above zero.Preferably, the current should fall below the sizing level, but still beabove at least 10 kA. The purpose of this step is two-fold. First, itallows for some cooling at the electrode/sheet interface. Second, itprevents the initiated weld nugget from overheating due to the highpower pulse applied during the Shaping step. Removing this short coolingstep typically leads to overheating of the nugget during the Sizing stepand severe interfacial expulsion of molten metal. Allowing the initiatedmolten nugget to cool helps bring the process under control and helpsstabilize the Sizing step. However, this cooling step cannot be so longthat nugget solidification occurs. If the nugget were to solidify itwould act as a short between the sheets, thus additional current flowduring the Sizing step would not achieve the desired weld size.

Step 3 Sizing: Once the weld nugget has been initiated in the center ofthe faying interface contact zone by the Shaping step and allowed tocool, the rms current level is adjusted to a level that is below thepeak current used for the Shaping step to achieve the target weld nuggetsize. This rms value is typically several kA below the peak currentattained in the Shaping step as shown in the Table I. It is also severalkA above the minimum current attained in the cooling step that followsShaping. Sizing current values (rms) are typically between 15 kA forlight gage aluminum alloy sheet to 40 kA for very heavy gauge sheet. TheSizing may be done with a constant current such that the peak and rmscurrent values are the same or, if electrode wear becomes an issue,sizing may be done with a series of current pulses with an rms valueseveral kA below the peak shaping current. Sizing is typically done witha current flow period of from 20 ms to 200 ms and is the longest part ofthe weld schedule. When current pulses are used each pulse is typicallycomposed of ten to thirty milliseconds of on time (heating) and one tofive milliseconds of off time (cooling).

The sizing step is typically much shorter than the times typically usedto produce a weld with a constant current schedule. Table I abovecompares the sizing steps for various combinations of 5754 aluminumsheet with those recommended by the Aluminum Association for the priorart constant continuous current (CCC) schedule. The weld times are muchshorter for our CSS schedules, particularly for heavier gauge materials.Shorter weld times result in steeper temperature gradients around theweld nugget that act to cool the nugget more quickly, producing a morerefined microstructure. The refined microstructure has been found to bemore resistance to fracturing under peel loading than microstructuresobtained using typical weld schedules.

In addition, the shorter times for nugget Shaping plus nugget Sizingresult in much less energy used to produce a weld nugget. In thisexample about 40% less energy is used to produce a weld nugget or buttonof acceptable size. This reduces thermal load on the MFDC transformerand weld gun components such as gun arms, cables, shunts.

As an example, FIG. 5 shows weld sizes for a 2.0-mm 5754 stackup weldedunder three different conditions using a CSS schedule. Weld time istaken at the start of the shaping step and thus includes both theshaping and sizing pulses. Notice that even after 15 ms weld nuggetshave been formed for all three conditions. The rapid formation of a weldnugget greatly eliminates variability in the process.

At a weld time of only 115 ms, weld size is ˜6 mm or larger. Robust sizehas been achieved at shorter times that for the constant continuouscurrent (CCC) schedule. This extends electrode life and refines weldmicrostructure. Undesired fracture modes occur only at very short weldtimes of 35 ms. At 65 ms and longer times, no undesired fracture modeswere observed, which is improved performance compared to the CCCschedule, FIG. 2.

In many situations for the resistance spot welding of aluminum sheetmetal workpieces it may be preferred to use welding electrodes likethose disclosed in U.S. Pat. No. 6,861,609 (Mar. 1, 2005) and U.S.patent applications #20100258536, 20090302009, 20090255908, 20090127232,20080078749. These electrodes have roughened or shaped welding facesthat have been found to be useful in resistance spot welding of aluminumand such electrodes perform well when the spot welding is performed inaccordance with weld schedules of this specification.

The above described practices of the invention are for purposes ofillustration and are not to limit the scope of the invention.

The invention claimed is:
 1. A method of forming resistance spot weldson aluminum-based alloy workpieces, the method comprising: forming astack of two or more aluminum-based alloy workpieces, the two or morealuminum-based alloy workpieces having faying surfaces at a resistanceweld site and opposing outer surfaces at the resistance weld site;pressing weld faces of opposing resistance weld electrodes against theouter surfaces of the two or more aluminum-based alloy workpieces at theresistance weld site; and while pressing the weld faces against the twoor more aluminum-based alloy workpieces, passing a first stage weldcurrent between the opposing resistance weld electrodes and through thetwo or more aluminum-based alloy workpieces at the opposing resistanceweld site for a first stage time of milliseconds, the first stage weldcurrent being ramped to a first stage peak current value that reduces anelectrical resistance between the weld faces to a stable resistancevalue and heats the two or more aluminum-based alloy workpieces forengagement with the weld faces without melting of the two or morealuminum-based alloy workpieces at their faying surfaces, and then,momentarily reducing current flow with the opposing resistance weldelectrodes still pressed against the outer surfaces of the two or morealuminum-based alloy workpieces; passing a second stage weld currentbetween the opposing resistance weld electrodes and through the two ormore aluminum-based alloy workpieces at the weld site for a second stagetime of milliseconds, the second stage weld current being ramped to asecond stage peak value, larger than the first stage peak current value,for initiating a molten weld nugget formation at the faying surfaces ofthe weld site, and then, reducing current flow with the opposingresistance weld electrodes still pressed against the outer surfaces ofthe two or more aluminum-based alloy workpieces to stabilize temperaturedistribution at the faying surfaces of the weld site; passing a thirdstage weld current between the opposing resistance weld electrodes andthrough the two or more aluminum-based alloy workpieces at the weld sitefor a third stage time of milliseconds, the third stage weld currentbeing at a third stage value, smaller than the second stage peak currentvalue, for completing the molten weld nugget formation of apredetermined weld nugget diameter at the faying surfaces of the weldsite, the third stage time being longer than the second stage time, andstopping current flow for cooling of the weld site and solidification ofthe molten weld nugget; and then removing the weld faces of theopposing-resistance weld electrodes from contact with the outer surfacesof the two or more aluminum-based alloy workpieces.
 2. The method offorming resistance spot welds on aluminum-based alloy workpieces asstated in claim 1 in which the weld faces of the opposing resistanceweld electrodes are convex and engage the two or more aluminum-basedalloy workpieces with intruding grooves, protruding ridges, or acombination of both, formed on the weld faces.
 3. The method of formingresistance spot welds on aluminum-based alloy workpieces as stated inclaim 1 in which the weld faces of the opposing resistance weldelectrodes are convex and engage the two or more aluminum-based alloyworkpieces with roughened surfaces.
 4. The method of forming resistancespot welds on aluminum-based alloy workpieces as stated in claim 1 inwhich the first stage weld current is slowly ramped to a value forremoving adhesive or other unwanted non-metallic material at the fayingsurfaces of the two or more aluminum-based alloy workpieces.
 5. Themethod of forming resistance spot welds on aluminum-based alloyworkpieces as stated in claim 1 in which the second stage weld currentis ramped to about twenty to about fifty kilo-amperes and the secondstage time lasts about six to about fifty milliseconds, the second stageweld current and the second stage time providing for formation of themolten weld nugget of the predetermined weld nugget diameter andcentered at a contact area of the weld site of the faying surfaces. 6.The method of forming resistance spot welds on aluminum-based alloyworkpieces as stated in claim 1 in which the third stage value is aboutfifteen to about forty kilo-amperes and the third stage time lasts for aperiod determined for the formation of a required size of the moltenweld nugget depending on a thicknesses of the two or more aluminum-basedalloy workpieces.
 7. The method of forming resistance spot welds onaluminum-based workpieces as stated in claim 1 in which the third stageweld current is passed between the opposing resistance weld electrodesas two or more pulses with heat times of up to thirty milliseconds andcool times of one to ten milliseconds.
 8. A method of forming resistancespot welds on aluminum-based alloy workpieces, the method comprising:forming a stack of two or more aluminum-based alloy workpieces, the twoor more aluminum-based alloy workpieces having faying surfaces at aresistance weld site and opposing outer surfaces at the resistance weldsite; providing a medium frequency direct current (MFDC) using aninverter type weld control that initially receives a three-phasealternating current of set frequency and rms voltage which is convertedto a single phase, medium frequency primary current, which is then fedto a MFDC transformer and rectifier to convert to a direct weldingsecondary current, pressing weld faces of opposing resistance weldelectrodes against the outer surfaces of the two or more aluminum-basedalloy workpieces at the resistance weld site; and while pressing theweld faces against the two or more aluminum-based alloy workpieces,passing a first stage direct weld current between the opposingresistance weld electrodes and through the two or more aluminum-basedalloy workpieces at the resistance weld site lasting for a first periodof milliseconds, the first stage direct weld current being increased toa first stage peak direct current value that reduces an electricalresistance between the weld faces to a stable resistance value and heatsthe two or more aluminum-based alloy workpieces for engagement with theweld faces without melting of the two or more aluminum-based alloyworkpieces at their faying surfaces, and then, momentarily reducing aprimary alternating current flow with the opposing resistance weldelectrodes still pressed against the outer surfaces of the two or morealuminum-based alloy workpieces; passing a second stage direct weldcurrent between the opposing resistance weld electrodes and through thetwo or more aluminum-based alloy workpieces at the resistance weld sitelasting for a second period of milliseconds, the second stage directweld current being increased to a second stage direct current value,larger than the first stage peak direct current value, for initiating amolten weld nugget formation at the faying surfaces of the resistanceweld site, and, when a molten weld nugget of predetermined size has beenformed, reducing the primary current flow with the opposing resistanceweld electrodes still pressed against the outer surfaces of the two ormore aluminum-based alloy workpieces; passing a third stage direct weldcurrent between the opposing resistance weld electrodes and through thetwo or more aluminum-based alloy workpieces at the resistance weld sitelasting for a third period of milliseconds, the third stage direct weldcurrent being at a third stage rms current value, smaller than thesecond stage peak direct current value, for completing the molten weldnugget formation at the faying surfaces of the resistance weld site, thethird period being longer than the second period, and, again, stoppingthe primary current flow for cooling of the resistance weld site andsolidification of the molten weld nugget; and then removing the weldfaces of the opposing resistance weld electrodes from contact with theouter surfaces of the two or more aluminum-based alloy workpieces. 9.The method of forming resistance spot welds on aluminum-based alloyworkpieces as stated in claim 8 in which the first stage direct weldcurrent is ramped to a value of about ten kilo-amperes and the firstperiod lasts about twenty to about sixty milliseconds; in which thesecond stage direct weld current is more quickly ramped to about twentyto about fifty kilo-amperes and the second period is about six to aboutfifty milliseconds, the second stage direct weld current and the secondstage period providing for formation of the molten weld nugget ofpredetermined size and centered between the faying surfaces of the twoor more aluminum-based alloy workpieces; and in which the third stagedirect weld current is about fifteen to about forty kilo-amperes and thethird period lasts for a period just required to form a weld nugget ofdesired size.
 10. The method of forming resistance spot welds onaluminum-based workpieces as stated in claim 8 in which the third stagedirect weld current is passed between the opposing resistance weldelectrodes as two or more pulses with heat times and cool timesdetermined for forming a weld nugget of predetermined size.